Notifications

Description

termscp will now send Desktop notifications in these cases
- on transfer completed (minimum transfer size can be specified in configuration; default 64MB)
- on transfer error (same as above)
- on update available
Added "notifications enabled" in configuration (Default enabled)
Added "Notifications: minimum transfer size": if transfer size is greater or equal than the specified value, notifications for transfer will be displayed.

Please select relevant options.

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[x] This change requires a documentation update

[x] My code follows the contribution guidelines of this project
[ ] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[x] My changes generate no new warnings
[x] I formatted the code with cargo fmt
[x] I checked my code using cargo clippy and reports no warnings
[x] I have added tests that prove my fix is effective or that my feature works
[x] I have introduced no new C-bindings
[x] The changes I've made are Windows, MacOS, UNIX, Linux compatible (or I've handled them using cfg target_os)
[x] I increased or maintained the code coverage for the project, compared to the previous commit

wait for a project maintainer to fulfill this section...

[x] when notifications are enabled and a transfer of size with at least configured threshold is completed, a notification is shown.
[x] when notifications are enabled and a transfer of size with at least configured threshold fails, a notification is shown.
[x] when notifications are enabled and a transfer of size less than configured threshold is completed, no notification is shown.
[x] when notifications are enabled and a transfer of size less than configured threshold fails, no notification is shown.
[x] when notifications are disabled and a transfer of size with at least configured threshold is completed, a notification is not shown.
[x] when notifications are disabled and a transfer of size with at least configured threshold fails, a notification is not shown.
[x] when notifications are enabled and an update is enabled, a notification is shown.
[x] when notifications are disabled and an update is enabled, a notification is not shown.
[x] when notifications are enabled and an update is installed, a notification is shown.
[x] when notifications are disabled and an update is installed, a notification is not shown.